Solvent type or water-soluble type acrylic/melamine resin coatings and acrylic/urethane resin coatings have widely been employed as coatings for vehicle bodies which are required to have a highly attractive appearance as well as other various coating performances. In recent years, photo-curable coatings and radiation-curable coatings have been proposed in addition to the above-described coatings curable by crosslinking. The photo-curing coatings include a composition comprising a modified isocyanate having an ethylenical double bond and an isocyanate group and a hydroxyl-containing prepolymer as disclosed in Japanese Patent Application (OPI) No. 66596/75 (the term "OPI" as used herein means "unexamined published application"). The radiation-curable coatings include a composition comprising an ethylenic polymer having a (meth)acrylic ester and a hydroxyl group, a vinyl polymer, and a photo initiator as basic components and, in addition, a reaction product between the basic components and a polyisocyanate compound as disclosed in Japanese Patent Application (OPI) No. 17967/79.
The aforesaid acrylic/melamine resin coatings and acrylic/urethane resin coatings do not necessarily exhibit satisfactory appearance and find difficulty in coating thickly. A high film thickness may be attained by increasing a non-volatile residue, i.e., total solids, of a coating. However, means generally taken to this effect, such as reduction of molecular weight of resinous components to decrease the viscosity, result in deterioration of workability of the coating and physical properties, chemical resistance, and weathering resistance of a coating film.
On the other hand, the photo-curable coatings or radiation-curable coatings are easily obtained high film thickness and their coating films are excellent in appearance. However, they show poor adhesion to a substrate due to great distortion upon curing. Moreover, sufficient curing cannot be achieved on areas where ultraviolet radiation is insufficient so that satisfactory weathering resistance cannot be always assured.